This invention relates in general to capillary electrokinetic devices and in particular to an improved system for measuring electrokinetic properties and for characterizing electrokinetic separation by monitoring current in an electrophoretic process.
In capillary zone electrophoresis (CZE) a migration channel of capillary dimensions is filled with electrolyte. A sample to be analyzed is injected at one end of the channel and a high voltage is applied across the channel, causing the sample in the electrolyte to migrate from one end of the channel to the other. When the electrolyte contacts the walls of the capillary, the inner surface of the capillary becomes charged, either through ionization of surface groups on the capillary walls or through the adsorption of charged species from the electrolyte onto the inner surface. In either case the electrolyte inside the capillary is no longer electro-neutral but acquires a net charge which may be positive or negative. Under the action of the applied electric field, the electrolyte moves towards one end of the capillary, and this movement is referred to as electroosmotic (electroendosmotic) flow.
While the electrolyte as a whole acquires a net charge so that the electrolyte as a whole flows toward one end of the capillary, different components of the sample may be positively or negatively charged.
Therefore in addition to the electroosmotic flow of the electrolyte, electrophoresis also takes place; that is, the applied electric field exerts a force on positively charged species to cause them to move to the negatively charged electrode and on negatively charged species to cause them to move to the positively charged electrode. As a result, the components in the injected sample separate into distinct zones, based on their different mobilities. Frequently, the rate of electrophoretic flow is less than the rate of electroosmotic flow. Consequently, species in the injected sample move in one direction--the direction of the electroosmotic flow--and thus the different species can be detected as each zone passes through some suitable detector located downstream from the capillary inlet.
Clearly, a precise characterization of the electroosmotic flow is highly desirable not only for understanding CZE but also for optimizing the operation of CZE in analyzing a given sample. One way to measure the electroosmotic velocity is to record the migration time of an injected uncharged marker solute, which will be carried through the capillary under the action of only electroosmotic flow. This technique is described in "Capillary Zone Electrophoresis of Neutral Organic Molecules by Solvophobic Association with Tetraalkylammonium Ion" by Walbroehl and Jorgenson, ANAL. CHEM. 1986, 58, 479-481. See also ((References 1 and 2)). First, it is very difficult to select a marker which is truly neutral. Furthermore, the charge on a particular marker selected would depend upon the medium in which is it placed. Therefore, an apparently "neutral" marker used in one medium may be unsuitable for use as a marker in the different medium. In addition, "neutral" markers may be difficult to detect.
Another method for measuring electroosmotic velocities is to weigh the mass of electrolyte transferred from the capillary inlet to capillary outlet over a timed interval. For a description of the method, see "Measurement of Electroendosmotic Flows in High-Voltage Capillary Zone Electrophoresis" by Altria and Simpson, ANAL. PROC. 1986, 23, 453-454. In this method, losses caused by evaporation must be eliminated by covering the surface of the liquid in the cathode compartment with a film of silicone fluid or by use of a tight-fitting lid. Since the mass of electrolyte transferred is minute, the use of a digital balance appears to be necessary. This method requires the measurement of minute mass transfers so that extra precautions must be taken to assure the accuracy of measurement so that it is inconvenient and cumbersome to use.
None of the above described method is entirely satisfactory. It is therefore desirable to provide an improved system for measuring electrokinetic properties of a solute in which the above described difficulties are alleviated.